Modems are used in point-to-point and multi-drop configurations. With increasing costs and data transmission burdens, the multi-drop approach has been used more frequently. In such configurations, when a transmitter or receiver fails in the "on" or short-circuit condition, the entire transmitter-receiver line goes down along with all its drops. The problem becomes more complex when one or more of the modems themselves supports another multi-drop line. When a line fails, it is often difficult to determine which modem is at fault and what is the basis of the problem. The modems may be in different locations in different parts of the country, and a modem may feed one or more additional modems. To help alleviate this problem, network controllers are introduced. However, they are quite expensive. They use side channels, frequency division multiplexed to interrogate modems and find which modem is at fault and diagnose the problem. The interrogation is done over the telephone line, but separately from the data. In cases where the first modem feeds other modems, the first modem must be provided with an interface or link to allow its other modems to be interrogated through it. One of the major shortcomings of network controllers is that they provide only the indication of the fault; a human operator must determine how to fix the fault. Correction is not normally automatic. Usually there is not a human monitor in continual attendance, so there may be a substantial interval between the time the fault is found and the time the human monitor sees the fault indication and moves to correct it. Further, the human monitor must be trained, knowledgeable person skilled in network problems.
Network controllers have been successfully used in larger systems where their cost is a small part of a much larger system, but their cost would overwhelm a small, simple, point-to-point system, especially those that are used in the less expensive, slower modems. In addition, the network controller itself may fail, and then it is truly difficult to determine the cause of the problem.